The present invention relates to the coupling of a light beam into an optical waveguide. More particularly, it concerns a device and method for the side-coupling of light from a light source into a waveguide, and is advantageously adapted to couple light from a laser diode array into one or a plurality of optical fibers.
Launching of a light beam into a waveguide such as an optical fiber is one of the most basic challenges of any type of optical systems. One of the most common methods of optical coupling is the fiber pigtailing longitudinally launching a light beam into an optical fiber from one of its extremities. Fiber pigtailing however requires a precise alignment of all involved components, and most importantly, is limited to cases where a fiber extremity is available for such a coupling.
To alleviate this latter drawback, several techniques for side coupling of a light beam into an optical fiber have been developed.
For example, known in the art is U.S. Pat. No. 4,815,079 (SNITZER et al.) which is particularly directed to optical fibers for lasers and amplifiers having a single-mode core, a multi-mode inner cladding surrounding this core and an outer cladding surrounding the inner cladding. FIGS. 3 to 5 of this patent show different manners of ways for launching a light beam for propagation into the inner core of the fiber. In each case, the outer cladding is removed along a coupling length, and a coupling fiber or other device is affixed adjacent to the inner cladding. The coupling fiber is disposed along the inner cladding longitudinally in FIG. 3, and at an angle in FIG. 5. In FIG. 4, a prism is used and also disposed with one of its sides adjacent to the inner cladding. In each cases, coupling is enabled through an appropriate selection of the refractive indices, dimensions and positioning of all components.
Referring to International patent application published under no. WO95/10868 (GASPONTSEV et al.), there is also shown an arrangement for side coupling of pump radiation from a multi-mode fiber into a double-core fiber, in this case, a multi-mode coupler is formed by twisting, heating and subsequently pulling the two fibers.
Also known in the art is U.S. Pat. No. 5,999,673 (VALENTIN et al.), which particularly addresses the side coupling of a beam from a light source into an optical fiber. VALENTIN discloses the use of an intermediate feeding fiber coupled to the light source at one end and to the destination optical fiber at the other end. The feeding fiber is tapered at this latter end and fused to the other optical fiber at this tapered portion.
The above systems all allow the side coupling of a light beam in an optical fiber through the use of an intermediate fiber. In the case of radiation pumping, the light source must already be fiber based or be coupled to this intermediate fiber with the corresponding drawbacks. It would therefore be advantageous to provide a technique that allows side coupling of pump radiation inside an optical fiber without the use of an intermediate fiber.
To this effect, U.S. Pat. No. 5,854,865 (GOLDBERG) discloses a method and apparatus for side pumping of an optical fiber. GOLDBERG teaches the provision of a transversal groove, preferably V-shaped, extending into the outer core of a double core fiber. A laser diode is disposed on the opposite side of the fiber and directs light transversally into the fiber. The light rays impinge on the facets of the groove and are specularly reflected inside the outer core along the longitudinal axis of the fiber, therefore launching the light from the diode to propagate into the outer core.
One drawback of GOLDBERG""s xe2x80x9cV-groovexe2x80x9d technique is that the focal length of the system is determined by the radius of the fiber, and that generally the various parameters of the coupler are determined by the fiber construction and geometry. In addition, this technique is practical only for coupling light from a single diode into the fiber, and is not adapted for coupling light from a plurality of sources such as the laser diodes of an array.
It is therefore an object of the present invention to provide a practical technique for the side coupling of a light beam into an optical fiber.
It is another object of the present invention to provide a coupling element adapted for realising such a technique.
It is a preferred object of the invention to allow side coupling of light from a laser diode array into one or a plurality of optical fibers.
In accordance with a first aspect of the present invention, there is therefore provided an optical coupling device for coupling light emitted by an array of laser diodes into at least one waveguide. This device includes a substrate, and a plurality of waveguide sections of the at least one optical waveguide affixed to the substrate in a parallel relationship with each other. Each of the waveguide sections has a light propagating layer extending along a longitudinal axis, and a groove extending therein transversally to this longitudinal axis. The groove reaches into the light propagating layer.
The device also includes a coupling element for coupling light from a plurality of laser diodes of the array into the waveguide sections. The coupling element has a first end optically coupled to the plurality of laser diodes for receiving the light therefrom, and a second end extending in the groove of each of the plurality of waveguide sections. The second end has at least one reflection interface oriented to reflect the light to propagate into the light propagating layer of the corresponding waveguide section.
According to another object of the invention, there is also provided a coupling element for coupling light emitted by a light source into a waveguide having a light propagating layer extending along a longitudinal axis and a groove extending therein transversally to the longitudinal axis, this groove reaching into the light propagating layer.
The coupling element includes a first end shaped to receive the light from the light source, and a second end shaped to be received inside the groove of the waveguide. The second end has at least one reflection interface oriented to reflect the light to propagate into the light propagating layer when the second end is inserted in the groove.
Finally, according to yet another aspect of the present invention, there is further provided a method for coupling light emitted by a light source into a waveguide having a light propagating layer extending along a longitudinal axis, the method comprising the steps of:
a) making a groove in a waveguide section of the waveguide transversally to the longitudinal axis, this groove reaching into the light propagating layer;
b) providing a coupling element for coupling light from the light source into the waveguide section, the coupling element having a first end and a second end having at least one reflection interface;
c) optically coupling the first end of the coupling element to the light source for receiving the light therefrom; and
d) inserting the second end of the coupling element in the groove of the waveguide section with the at least one reflection interface oriented to reflect the light from the light source to propagate into the light propagating layer of the waveguide section.
Further aspects and advantages of the present invention will be better understood upon reading of preferred embodiments thereof with reference to the appended drawings.